DISTINCT CIRCADIAN TIME STRUCTURES CHARACTERIZE MYELOID AND ERYTHROIDPROGENITOR AND MULTIPOTENTIAL CELL CLONOGENICITY AS WELL AS MARROW PRECURSOR PROLIFERATION DYNAMICS

Circadian differences in the susceptibility of the marrow to the effec ts of radiation, myelotoxic drugs, and growth factors suggest that hem atopoietic processes vary significant throughout each day. One mechani sm possibly responsible for the differing degrees of marrow damage sus tained from a fixed dose of a cytotoxic agent at different times of da y is the circadian organization of cell cycle events. Previous circadi an rhythm-oriented studies of proliferation using unfractionated marro w have reported seemingly contradictory peak and nadir times of day. M arrow represents a heterogeneous population of stem cells and various hematopoietic progenitors whose proliferation and differentiation are controlled by both common and unique factors. Therefore, we examined l ineage-specific circadian marrow proliferative dynamics for evidence o f parasynchronous circadian DNA synthesis. Cell cycle phase was determ ined using flow cytometry with both propidium iodide staining and 5-br omo-2'-deoxyuridine (BrdU) incorporation concurrently with cell cultur e-based determinations of lineage-specific progenitor numbers in the s ame marrow samples. Although no clear circadian (24-hour) rhythm chara cterized unfractionated marrow DNA synthesis, both erythroid- and myel oid-enriched subpopulations demonstrated distinct circadian patterns w ith respect to the percentage of cells incorporating BrdU, with up to 50% differences throughout each day. Interestingly, these circadian rh ythms in erythroid and myeloid progenitor cell DNA synthesis are entir ely different from one another. The lineage-specific circadian pattern s in the fraction of cells undergoing DNA synthesis are, in part, para lleled by up to eightfold larger circadian differences in erythroid an d myeloid colony numbers. Multipotential colony numbers likewise vary throughout the day, with a unique pattern of their own. The predominan t period length of daily rhythms in colony numbers and their amplitude s differ as a function of the stage of progenitor commitment. Multipot ent and early progenitor colony numbers each exhibit 24-hour rhythms, with three-to fivefold daily peak-trough differences, whereas later pr ogenitor colony numbers exhibit two peaks per day (12-hour rhythms) wi th twofold peak-trough differences throughout each day. In vivo erythr opoietin (Epo) administration enhances daily rhythms in erythroid colo ny numbers by increasing their amplitudes while leaving their circadia n shapes virtually unchanged. The increment in erythroid colony number s after Epo administration varies up to 16-fold with the time of day o f treatment. In summary, we have defined distinctly different lineage- dependent circadian patterns of marrow progenitor numbers and prolifer ating cells. We can infer from these data that the circadian timing of administration of physical, chemical, or biologic agents, whose bioac tivity toward marrow precursors depends on the cell cycle phase of its presentation, can be expected to affect this activity predictably and significantly. These results may have practical applications in impro ving stem and progenitor cell yields by optimal circadian timing of gr owth factor administration and harvest.